In lighting equipment using LEDs, AC power supplied from a commercial power supply is full-wave rectified by a diode bridge, and the rectified output voltage is applied across a plurality of series-connected LEDs, causing the plurality of LEDs to emit light.
LEDs have nonlinear characteristics such that, when the voltage being applied across the LED reaches or exceeds its forward voltage drop (Vf), current suddenly begins to flow. Desired light emission is produced by flowing a prescribed forward current (If) by a method that inserts a current limiting resistor or that forms a constant current circuit using some other kind of active device. The forward voltage drop that occurs here is the forward voltage (Vf). Accordingly, in the case of a plurality, n, of LEDs connected in series, the plurality of LEDs emit light when a voltage equal to or greater than n×Vf is applied across the plurality of LEDs. On the other hand, the rectified voltage that the diode bridge outputs by full-wave rectifying the AC power supplied from the commercial power supply varies between 0 (v) and the maximum output voltage periodically at a frequency twice the frequency of the commercial power supply. This means that the plurality of LEDs emit light only when the rectified voltage reaches or exceeds n×Vf (v), but do not emit light when the voltage is less than n×Vf (v).
To address this deficiency, it is known to provide an LED driving circuit in which a plurality of LEDs are blocked into four groups (3-1 to 3-10, 3-11 to 3-20, 3-21 to 3-30, and 3-31 to 3-40) and a switching device for connecting each LED group to a rectifier is controlled in accordance with the output voltage of the rectifier (refer, for example, to patent document 1).
This prior known circuit, however, requires the provision of a switch circuit for switching the connection mode of the plurality of LED blocks, and the switch circuit can only be controlled by a method that switches the connection based either on a comparison between the rectified voltage and the output of a current detector or on the rectified voltage. Therefore, with this prior known LED driving circuit, it is not possible to set proper switching voltage in advance by using an economical method, and there has therefore been the problem that not only does the overall size and cost of the LED driving circuit increase, but the power consumption also increases because of the power required to drive the switch circuit. In particular, if the light-emission period of the LEDs is to be further lengthened, the number of LED blocks has to be increased, but if the number of LED blocks is increased, the number of switch circuits required correspondingly increases.
Further, the switching timing of the switch circuit is set based on the predicted value of n×Vf (v), but since Vf somewhat varies from LED to LED, the actual value of n×Vf (v) of each LED block differs from the preset value of n×Vf (v). This has led to the problem that even if the switch circuit is set to operate in accordance with the supply voltage, the LEDs in the respective blocks may not emit light as expected, or conversely, even if the switching is made earlier than the preset timing, the LEDs may emit light; hence, the difficulty in optimizing the light-emission efficiency and the power consumption of the LEDs.
It is also known to provide a method in which a plurality of LED blocks, each containing a plurality of LEDs, are connected in series and are controlled on and off in an efficient manner in accordance with the rectified voltage output from a full-wave rectifier (refer, for example, to patent document 2).
FIG. 13 is a diagram schematically illustrating the configuration of the prior known LED driving circuit 200 disclosed in the above patent document 2. The prior known LED driving circuit 200 will be described below with reference to FIG. 13.
In the LED driving circuit 200, LED blocks Gr1 to Gr5, each containing a plurality of LEDs, are connected in series to the full-wave rectifier 202. The LED driving circuit 200 further includes circuits 231 to 235 corresponding to the respective LED blocks Gr1 to Gr5. Further, the LED driving circuit 200 includes comparators CMP1 to CMP3 and OR circuits OR1 and OR2 which are used to turn off the LED blocks Gr1 to Gr3.
The circuits 231 and 232 perform control to maintain the sum of a drain current IQ1, which flows from the LED block Gr1 to an nMOSFET Q1, and a drain current IQ2, which flows from the LED block Gr2 to an nMOSFET Q2, constant. As the rectified output voltage of the full-wave rectifier gradually increases from a voltage just sufficient to cause the LED block Gr1 to emit light to a voltage sufficient to cause the LED blocks Gr1 and Gr2 to emit light, the drain current IQ2 begins to flow. Here, if the drain currents IQ1 and IQ2 are allowed to flow freely, the current flowing through the LED block Gr1 may exceed the allowable amount; therefore, the circuits 231 and 232 perform control to maintain the sum of the drain currents IQ1 and IQ2 constant. That is, when the rectified output voltage of the full-wave rectifier reaches the voltage sufficient to cause the LED blocks Gr1 and Gr2 to emit light, control is performed to block the drain current IQ1 and to allow only the drain current IQ2 to flow. In this condition, the LED blocks Gr1 and Gr2 are connected in series to the full-wave rectifier, and the LEDs contained in the LED blocks Gr1 and Gr2 emit light.
Similarly, when the rectified output voltage of the full-wave rectifier reaches a voltage sufficient to cause the LED blocks Gr1 to Gr3 to emit light, the circuits 232 and 233 perform control so as to block the drain current IQ2 and to allow only the drain current IQ3 to flow. In this condition, the LED blocks Gr1 to Gr3 are connected in series to the full-wave rectifier, and the LEDs contained in the LED blocks Gr1 to Gr3 emit light.
When the rectified output voltage of the full-wave rectifier further rises and reaches a voltage sufficient to cause the LED blocks Gr1 to Gr4 to emit light, the circuits 233 and 234 perform control so as to block the drain current IQ3 and to allow only the drain current IQ4 to flow. In this condition, the LED blocks Gr1 to Gr4 are connected in series to the full-wave rectifier, and the LEDs contained in the LED blocks Gr1 to Gr4 emit light.
When the rectified output voltage of the full-wave rectifier further rises and reaches a voltage sufficient to cause the LED blocks Gr1 to Gr5 to emit light, the circuits 234 and 235 perform control so as to block the drain current IQ4 and to allow only the drain current IQ5 to flow. In this condition, the LED blocks Gr1 to Gr5 are connected in series to the full-wave rectifier, and the LEDs contained in the LED blocks Gr1 to Gr5 emit light.
In this way, the circuits 231 to 235 perform control so as to maintain the sum of the drain currents constant by sequentially blocking the drain current flowing in each circuit on the downstream side (the full-wave rectifier side).
However, for example, when the drain current IQ2 is blocked, and the drain current IQ3 is allowed to flow, if the drain current IQ1 begins to flow, a large current will flow through the LED block Gr1, which is not desirable. Therefore, when the drain current IQ3 flows, control is performed to set the output of the comparator CMP1 high and thereby send a control signal via OR1 to the circuit 231 so that the drain current IQ1 can be blocked in a reliable manner.
Similarly, when the drain current IQ4 flows, control is performed to set the output of the comparator CMP2 high and thereby send a control signal via OR1 and OR2 to the circuits 231 and 232 so that the drain currents IQ1 and IQ2 can be blocked in a reliable manner.
Further, when the drain current IQ5 flows, control is performed to set the output of the comparator CMP3 high and thereby send a control signal via OR1 and OR2 to the circuits 231 to 233 so that the drain currents IQ1, IQ2, and IQ3 can be blocked in a reliable manner.
As described above, in the prior known LED driving circuit 200, each time an additional LED block is connected in series to the full-wave rectifier 202, control must be performed so that the current does not flow from any of the currently connected LED blocks directly to the full-wave rectifier. For example, consider the situation where the current is flowing with the LED blocks Gr1 to Gr4 connected in series to the full-wave rectifier, and the LED block Gr5 is additionally connected in series to the full-wave rectifier; in this case, the drain currents IQ1 to IQ3 are controlled digitally so that the currents do not flow from the respective LED blocks Gr1 to Gr4 directly to the full-wave rectifier 202, and the drain current IQ4 is controlled in analog so that the sum of the drain currents IQ4 and IQ5 is maintained constant.
In this way, when connecting a maximum number, N, of LED blocks in series to the full-wave rectifier, the prior known LED driving circuit 200 requires the provision of control circuitry that performs control to block the currents flowing from the (N−1) LED blocks to the full-wave rectifier. This has led to the problem that the digital control circuit becomes complex, increasing the size and cost of the entire circuitry.    Patent document 1: Japanese Unexamined Patent Publication No. 2006-244848 (FIG. 1)    Patent document 2: Japanese Unexamined Patent Publication No. 2010-109168 (FIG. 1)